Photonic switches capable of routing wide-band optical signals are a vital element of high capacity fiber-optic networks. A photonic switch generally comprises a multi-stage connecting network, each stage including switching devices and controllers, which routes optical information between input and output ports. Data flow bottlenecks occur in such switching networks when, for switching purposes, it is necessary to convert signals in optical form to electronic signals and then to reconvert them back to the form for retransmission. It is desirable to maintain the optical signals in optical form throughout the switch and to control the pathway through the switch either electronically or optically.
Electronically controlled, photonic switches have been demonstrated which include 2.times.2 integrated-optic, waveguide devices. Arrays of these 2.times.2 devices have been organized in N.times.N crossbar and other configurations. For instance, see "Waveguide Electrooptic Switch Arrays", Alferness, IEEE Journal On Selected Areas of Communications, Volume 6, Number 7, August 1988, pages 1117-1130. Optically controlled photonic switches have also been developed recently.
Whatever a photonic switching device is used, the speed of the switch is limited not only by the device's switching speed but by the speed of its control processor, which can, under certain circumstances, create severe data flow bottlenecks. These bottlenecks can be eliminated if optical processing is used to control the switch. If an optical processor is used with electro-optic switches requiring electric control, the processor's output must be converted to electrical signals to control the switch. The speed of such a switch is then limited by the speed of the optical decision-making process or the speed of the photonic switching device itself.
Optical control of a photonic switching element has been previously disclosed by the Inventors in "Photonic Switch With Optically Self-Routed Bit Switching" Prucnal et al., IEEE Communications Magazine, May 1987, Vol. 25 No. 25, pages 50-55. In that system, the data source encodes destination information in each data bit by using an optical spread-sprectrum technique. Each bit is encoded with a code sequence of N signals representing the destination address of that bit. An optical filter correlates its own stored address with the received encoded signal and provides an autocorrelation function output if the signal is to be switched. A crosscorrelation function indicates that the signal is not to be switched. One limitation of this technique arises from the need for orthogonal code sequences that can be distinguished at the correlator. Thus, to utilize the spread-spectrum technique, substantial bandwidth is required. In other words, for every N addresses, N.sup.2 time slots are required to generate N distinguishable code sequences.
Accordingly, it is an object of this invention to provide a photonic switch wherein the switching function is controlled by a bandwidth-conserving addressing technique.
It is a further object of this invention to provide a photonic switch which is capable of being implemented with either electronic or optical control circuitry.
It is still another object of this invention to provide a self-clocked, optically controlled, photonic switch which does not require the conversion of the message being switched from optical to electronic manifestations.